text:
What is the propeller efficiency, $μ_p$, of modern propellers for light sport aircraft?
Roskam's book on preliminary design gives a value of 0.7 for "homebuilt" aircraft and 0.8 for general aviation. What explains this difference in propeller efficiency? The content of Roskam's book is fantastic but some of the constants that he gives may be a bit dated as was the case for specific fuel consumption which was given as 0.7 for homebuilt aircraft when a modern engine like the Rotax 912UL has a sfc of 0.47.
On a search of Wikipedia, it is written that modern propellers can have an efficiency of 0.9.

Propeller speed is the answer. Many homebuilds use converted car engines which run at higher speeds than the aviation-certified gasoline engines.

Propeller charts assume some fuselage shape and somespinner shape, and these are important factors to consider. Especially if the configuration is a pusher-prop type.

Propeller efficiency is mentioned a lot here on Aviation SE, but lacks a good explanation. Here we go:
A propeller accelerates the air of density $\rho$ which is flowing through the propeller disc of diameter $d_P$. This can be idealized as a stream tube going through the propeller disc. This stream tube starts with air at ambient speed and a wide diameter d$_0$. As the suction ahead of the propeller accelerates the air in the streamtube, it starts to speed up and the stream tube contracts. In the propeller plane we witness a jump in pressure p: Here, energy is added to the flow, so Bernoulli does not apply momentarily. Past the propeller the air in the stream tube accelerates further and the tube contracts to the diameter d$_1$ when the pressure inside has dropped to the ambient pressure (dashed line in the pressure graph below):

The air speed ahead is $v_0 = v_{\infty}$ and the air speed aft of the propeller is $v_1 = v_0 + \Delta v$. The propeller effects a pressure change which sucks in the air ahead of it and pushes it out. Since the mass flow must be equal ahead and behind the propeller, the stream tube diameter is bigger ahead of the propeller and smaller downstream. In reality, there is no neat boundary between the air flowing through the propeller and that surrounding it, but for computing thrust this simplification works well if the airspeed is identical across the cross section of the propeller disc.
The efficiency $\eta$ of thrust creation is the work done on the mass flow through the propeller $W = m\cdot\Delta v\cdot v_0$ relative to the impulse change of the air $\Delta I = m\cdot\frac{v_1^2 - v_2^2}{2}$:
$$\eta^{opt} = \frac{2\cdot v_0}{v_1 + v_0} = \frac{v_{\infty}}{v_{\infty} + \frac{\Delta v}{2}} = \frac{1}{1 + \frac{\Delta v}{2\cdot v_{\infty}}}$$
This equation assumes that air is uniformly accelerated straight backwards. To be more precise, you need to add the swirl losses, since the air receives a rotational component $\omega$ from the propeller, spinning with the angular velocity $\Omega$, as well:
$$\eta^{opt}_{Prop} = \frac{1 - \frac{\Delta v\cdot \left({v_{\infty} + \frac{\Delta v}{2}}\right)}{d_P^2\cdot\Omega^2}}{1 + \frac{\Delta v}{2\cdot v_{\infty}}}$$
Still, we have not included friction losses yet and our prop spinner and engine nacelle are also not included. Now we must enter into a definition what propeller thrust is: Is it just the lift acting on the propeller blades in forward direction, or is it the remaining forward force after the additional drag of the airplane components in the slipstream of the propeller has been subtracted?
To avoid lengthy computations, charts can be used where the efficiency is plotted over a range of parameters.
To cut the discussion short: Generally it is safe to assume a top propeller efficiency of 0.85 (85%) with big, slowly spinning propellers (1000 to 1700 RPM). If the twist distribution along the blade does not match the local angle of attack distribution (say, if the propeller is optimized for high speed, but operated at slow airspeed, like during take-off), efficiency can easily drop to 0.7 (70%). Things become worse if the pitch of the blades is fixed. See below for a typical example of a variable pitch propeller. Each of the curves is for a different pitch setting, the x axis shows the advance ratio (the ratio between airspeed and circumferential speed; here off by a factor of 1/$\pi$), while the y axis shows the efficiency.

Efficiency chart of a variable pitch propeller. Source: McCormick B.W. Aerodynamics, Aeronautics & Flight Mechanics. John Wiley & Sons, Inc., 1979.
What can be seen from the equations above is that it is more efficient to accelerate lots of air a little than to accelerate a little air by a lot. This means that small propellers on ungeared engines, spinning at high RPM, are at a distinct disadvantage; this is why Roskam assumes only 70% for them and only 80% for GA propellers.
The 90% you name has to my knowledge only been scratched at by some very efficient (slow, large, contra-rotating) propellers operating under ideal conditions. To play it safe, I would choose a slightly smaller number for them.

Why is d0 is bigger than dp and moroever than d1? Without any explanation, I thing the depiction is lead to misunderstanding. Is any explanation about the d0, dp, and the d1?

@AirCraftLover Continuity. The same mass streaming along at a higher speed needs less space to do so. The change in diameter is inversely proportional to the square of the change in speed. Think of air flowing inside flexible tubes with infinitely thin walls - as the air speeds up, the tube will become narrower, at least in subsonic flow.

@PeterKämpf This is a great answer. When you say "Still, we have not included friction losses yet...", if I wanted real-world efficiency of the prop and thus to take friction into account, roughly what percentage ranges are typical? Is 90% a fair value for a well-designed blade, or is it a little higher/lower?

@KennSebesta This is a great question! Really, because looking at the propeller alone will not give a meaningful answer. The best reference I know is an article about measuring thrust on a Luscombe-8E, and there overall efficiency was abut 62%. The big losses are from the accelerated slipstream which raises fuselage drag. Of course, the low Reynolds number contributes to the high losses. See here for the reference.

@KennSebesta If you look at the blade alone, friction can be neglected on highly loaded propellers, but since the highest efficiency is found on slow, large props, friction has a higher share on very efficient props. Do you have access to XROTOR? There you can input factors for the viscous drag calculation which will help to isolate friction effects.

@PeterKämpf I can play around with XROTOR, but not in any intelligent manner where I would know if the coefficients I get are reasonable for the prop I have. I'm doing some optimization math for a glider self-launch and so just need the order of magnitude so I can see what the dominant effects are.

@PeterKämpf Inspired by your response, I opened https://aviation.stackexchange.com/questions/92975/what-is-the-range-of-ratios-between-optimal-propeller-efficiency-and-true-propel

It's quite low... After a series of gliding experiments made with a Luscombe 8E, and published by the AIAA, the conclusion was that the prop efficiency was around 62%...
https://engineering.purdue.edu/~andrisan/Courses/AAE490A_S2010/Buffer/AIAA-46372-872.pdf

Upvoted just for the link to the paper. By the way, the prop efficiency is 81% while the overall efficiency drops to 62% due to slipstream losses. The drag in powered flight is 30% higher from prop-airframe interaction!

Another major factor is the propeller design, profile, and RPM. This is limited by the material the blade is made from.
For example constant speed props typically achieve 10% better efficiency than fixed pitch. Also wood props are about 5% less efficient than the equivalent metal prop because the metal prop can be made with a longer and thinner airfoil.
The most efficient prop I have ever seen was rated at 92% for a Mooney. Aviation technical books typically rate wood fixed pitch at about 65-70% and metal at 70-75%, followed by constant speed at 80-85%.
 The Science of Flight, W N Hubin - 1992
 Design for Flying, David Thurston - 1978

Wood fixed pitch at about 65-70%, metal fixed propeller at 70-75%, followed by constant speed at 80-85%. What is this constant speed prop made of? Is it not possible constant speed also made of wood? Are you saying the constant speed here is always made of metal?

Efficiency changes as the aircraft speed changes, all else being held the same (see the figure above - the horizontal axis is advance ratio = V/ND).   D is prop diameter, fixed. N is engine speed, which maybe varies 25% from cruise to full power. V varies from 0 to Vne.
So, knowing a single exact number for efficiency is unhelpful for design, especially of fixed pitch props, which only get their max efficiency at one value of advance ratio.  It would be crazy to use a value of 0.9 in design, and then discover your plane won't fly unless it achieves Vne before lift off.  A plane - especially a low drag one - needs max prop power in the climb phase when it is slow (low advance ratio) and efficiency is low.  Use conservative values of efficiency, or your plane will fly fast but won't climb.


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What is the Extent of the Emergency Authority of the Pilot-in-Command (FAA)?
When writing an answer to this question regarding flight deck access during flight, I mentioned the authority of the PIC to deviate from regs as necessary for safety purposes, citing the usual FAR, 14 CFR 91.3(b), which says:

In an in-flight emergency requiring immediate action, the pilot in command may deviate from any rule of this part to the extent required to meet that emergency.

However, the exact wording used here made me wonder exactly how far this reg has been interpreted to go. In particular, it grants the PIC emergency ability to deviate from "Any rule of this part", which would be 14 CFR Part 91. So, does this authority extend only to Part 91 of the FARs or does it extend to all of the FARs? The exact wording seems to suggest it only applies to Part 91 and not, say, Part 121, which was the part under consideration in the other question.
So, the major things I'm wondering are:

What is the extent of the authority granted by 14 CFR 91.3(b)? and
If 14 CFR 91.3(b) indeed only applies to Part 91, is there a similar explicit emergency PIC authority exception to the other Parts of the FARs?

Obviously, the PIC is the one in charge and will deviate where necessary for safety regardless of regs and worry about the explaining later after he's ensured he'll be alive to explain, but I'm curious about the regulatory basis for deviation from FARs.

The scope of the part 91 exception is exactly what it says: deviate from any rule of this part, i.e. part 91. But remember that part 91 applies to all flights anyway; part 121 (or 135 etc.) regulations are in addition to part 91, not instead of them.
But that still means that 91.3(b) doesn't give any authority to deviate from other parts, like 135 or 121 regulations. 14 CFR 135.19(b) is the equivalent authority for part 135 operations:

In an emergency involving the safety of persons or property, the pilot
  in command may deviate from the rules of this part to the extent
  required to meet that emergency.

Note that the language is again of this part. So in an emergency, the PIC of a part 135 flight can deviate from part 91 rules per 91.3 and from part 135 rules per 135.19.
Part 121 is a little different. 14 CFR 121.557(a) says:

In an emergency situation that requires immediate decision and action
  the pilot in command may take any action that he considers necessary
  under the circumstances. In such a case he may deviate from prescribed
  operations procedures and methods, weather minimums, and this chapter,
  to the extent required in the interests of safety.

Here, the regulation allows for deviations from any part of this chapter, not this part as in the part 91 and 135 regulations. Chapter I of title 14 is at a higher level: it includes all parts of the FAA's aviation regulations, so under part 121 emergency authority the PIC can deviate from any aviation regulation, not just the ones in part 91 or 135.

Some other sections can also apply to GA flights (e.g. part 99 about ADIZes), but they also sometimes have their own emergency exceptions (e.g. 99.5)

Part 91 doesn't quite apply to all flights. It doesn't apply to ultralight aircraft (which are governed by part 103 instead), certain model aircraft (part 107), and certain special types of aircraft such as moored balloons amateur rockets (part 101).

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text:
Will increasing the wing area result in higher aircraft range?
Intuitively, it seems that a low loading (large area) wing will allow to climb higher from the start, thus increasing true air speed and range.
At the same time, a low loading wing also calls for a slightly lower cruising speeds due to higher lift produced, and has higher drag.
With all other things being equal (same fuselage, same amount of fuel, same engines) will increasing the wing area alone (by increasing the aspect ratio of example) result in higher aircraft range?

Up to a point.
There is an optimum for wing area that shifts with operational considerations and airfoil type.
Propeller aircraft benefit little from flying higher, and due to the high efficiency of piston engines, the highest wing loading possible with the desired take-off distance will give the best result. Aspect ratio should be as high as practical; past examples used values up to 14.5 for achieving high range.
Jets benefit more from altitude but need more fuel, so my main focus will be on jets. Now it helps to look at the Breguet range equation: $$R = \frac{v\cdot L/D}{g\cdot b_f} \cdot ln\left(\frac{m_1}{m_2}\right)$$
The indices denote the state at the beginning (1) and the end (2) of the flight. For a higher range $R$ it helps to increase

Fuel fraction $\frac{m_1}{m_2}$: The higher, the better. But note that doubling fuel will not double range. Bigger wings will allow to hold more fuel, so here wing size helps.
Flight speed $v$: This allows to cover more ground while in the air. However, flying faster will produce more drag and consume more fuel once you fly faster than at minimum drag.
Lift-to-drag ratio $L/D$: Obviously, producing less drag for the same lift is equally important. In order to shift the best L/D to higher speeds, a high wing loading helps because friction drag grows less than linearly with dynamic pressure.
Engine efficiency: Less fuel for the same thrust will let you fly longer. $b_F$ is the thrust-specific fuel consumption. Since this factor is independent of wing area, it can be disregarded for this answer. Indirectly, however, it helps to have more efficient engines because then the need for a high fuel fraction is less pressing. The inefficiency of their engines is the main reason for the low wing loading of the first generation of transatlantic jets.

An increase in wing area allows for a higher fuel fraction, as long as installed engine performance is sufficient to power the larger, heavier airplane. Both the higher mass and the higher wing area will increase drag, so the optimum range speed will decrease for increasing wing area. The tipping point where increasing area further will no longer yield an increase in range is reached when the fuel fraction is already so high that increasing it further buys too little range to make up for the lower cruise speed and the higher fuel flow enroute. For most configurations, the lower power loading of the larger, heavier airplane with the same engines will most likely set the upper limit of wing size.
The actual optimum shifts with details in the airplane's layout and operation. Supercritical airfoils with their higher relative thickness at transsonic speed and more efficient engines will directly help to increase range for the same wing area. The requirement for low altitude flight and not having to take off and land can also influence the result.
Don't neglect scale: Larger airplanes have disproportionally longer ranges than otherwise identical, smaller types.

Hi Peter, thanks. Just in case, I reworded the title, to better clarify my main question (range increase with larger area).

@ElectricPilot Acknowledged. I've rewritten part of the answer. Still, an analytical answer will need more details and be different for each airplane.

Increasing the wing area will reduce the wing loading and so slower flight will be possible, and parasitic drag will be reduced.  If the increase in wing area is achieved by increasing the aspect ratio then the induced drag will
also decrease.  The downside is that the aircraft will be slower, although that could be mitigated by using a thinner wing section and/or lower angle of attack.  The short answer appears to be yes, increasing wingspan alone reduces parasitic drag because there’s less pressure differential between upper and lower wing surfaces.

Hi Frog, thanks. Just in case, I reworded the title, to better clarify my main question (range increase with larger area)

Note that for a given airspeed, induced drag will decrease even if you keep the wingspan constant, (decreased aspect ratio!) simply because you increased area so decreasing angle of attack

Agreed Abdullah, induced drag is caused by the faster-moving and lower-pressure air above the wing interfering with the slower-moving and higher-pressure air below.  A high aspect ratio means that less of the wing is influenced by the tip vortices, but an increased chord means that the difference in pressure and speed is reduced.  In an extreme case, though, parasitic drag would become significant.

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text:
Does a quadcopter have flight modes beyond traditional aircraft?
Quadcopters, by virtue of software-piloting rather than human-piloting, are capable of new modes of flight. I've seen one in which the machine maintains altitude despite the loss of two rotors by spinning (yaw).
I wonder about a different mode, though; if a quadcopter could maintain a very high degree of pitch, then the flight is closer to a very short 4-engine X-wing aeroplane. This would be the advantage of a tiltrotor; after takeoff, rotate into the near-vertical position and permit much higher rates as we are no longer limited by usual helicopter dissymetry of lift arguments.
Is this possible? Are there other modes beyond the capacities of traditional aircraft? E.g. yaw spinning in the forward pitch position, coast-and-burn by varying the pitch periodically to switch between gaining altitude and gaining forward momentum, etc.

I'd imagine the control to be similar to a tandem rotor helicopter

All modes of flight of a quadcopter are beyond that of a traditional aircraft. Traditional aircraft have wings, no rotors. At sustained very high pitch the quadrotor will accelerate and then crash because of the lack of lift.

flitetest have a review of a quadcopter/airplane hybrid: http://flitetest.com/articles/Quadshot

As DeltaLima noted, pretty much all flight modes of a quadcopter are "non-traditional" -- they're a relatively new aircraft design.
Most quadcopter designs lack anything resembling "wings", so they can't "fly" in an "X-Wing" orientation (with the rotors producing entirely lateral thrust) -- at least not as we traditionally define flight.
They can, as you surmised, do a climb-and-run maneuver in that orientation: Gain a lot of altitude by using vertical thrust, tilt the platform near-90-degrees in the direction you want to travel (so your thrust is pretty much lateral), and accelerate in that direction while rapidly losing altitude.
I've no idea if this is more or less efficient than the usual slight tilt they fly with, but it's certainly less stable since you're in vertical free-fall and would need to redirect thrust to be vertical again with enough time/altitude to arrest the descent before an ungraceful impact with the ground.

X-Wings work so well because the microgravity environment of deep space isn't trying to drag you back to an impact with the ground - it wouldn't work well deep in a planet's gravity well, which is where most of our aircraft operate.
  Of course if you had access to a microgravity environment with an atmosphere you could configure a quadcopter to fly like an X-Wing fighter…

The quadcopter's thrust configuration does open up the option for some interesting maneuvers though.
It's theoretically possible to "flip" a quadcopter (around at least 8 axes) if you have sufficient altitude (especially if the rotors can be reversed in flight to assist in the maneuvers).
It's also possible to "rock the wings" (really rocking the whole platform) in many directions by alternating thrust on pairs of rotors.

There are UAVs that are designed to do just what you are describing.  I have no idea about how efficient they are.  But you can take a look at the Parrot Swing as one example.  It can take-off and hover as a quadcopter, but can transition to an airplane.

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Does the angled parking scheme of London City (LCY) increase the number of aircraft that can be parked in the small apron?
While going through some articles and videos, I chanced upon a video on LCY and was surprised to see that, unlike in Dortmund, LCY has aircraft parked diagonally. Does this increase the number of bays that the airport can have in that little space, or is there any other reason other than the one mentioned here?

The main reason for the diagonal parking is so that the airplanes can taxi up to and away from the stands without tugs. LCY has a lot of airplane movements for a small space so the parking is designed around getting airplanes in and out as quickly as possible: airplanes taxi in and park diagonally away from the terminal building so when they are ready to leave they power straight out to the taxiway. Note this is only possible because they don't have any jetways, passengers walk to and from the airplanes.

note for the curious: ALC also has this system for its remote stands. AMS on the other hand has them so aircraft can drive straight through most remote stands so don't need a tug either.

I like flying from LCY, you can show up 40 minutes before your flight and make it easily with time to spare.

... unlike AMS, where the plane has to leave the gate 40 minutes before the flight :( Seriously, it's such a small country, how can you spend so much time driving through fields?

My airport has jetways and the airplanes taxi out under their own power. Of course that's possible because there are (count 'em) a whole TWO jetways, and they're around the corner of the terminal from each other, so there's nothing in the way as they power up and taxi to their right.

@MSalters That's only if you fly with the cheaper airlines that get sent all the way to the Polderbaan. Schiphol typically always uses a second runway for departures together with the Polderbaan, so half of the departing flights skip the scenic trip. (But yeah, sometimes you might as well stay on the train and fly from Rotterdam instead.)

The advantage of diagonal parking at LCY is that it requires a narrower portion of the tarmac thereby allowing for adequate area for the width of the taxi-track.
The terminal charts Ground Movement instructions emphasize that final parking and taxiing out can only be done with guidance from  Marshaller - this indicates that standard clearances for wingtips and other extremities may not always be available, i.e. 'pilot judgement' alone, from the cockpit, is not enough to ensure safe maneuvering.
Her are some extracts from the eAIP for London City Airport:

Pilots are requested to use minimum power when manoeuvring on and off
parking stands . . . .
Parking: Pilots should
self-manoeuvre their aircraft on to the appropriate stand lead-in line
(as directed by ATC) and approach the stand as closely as possible.
Direction to the final parking position will be provided by
marshallers. Under no circumstances may aircraft enter a stand without
guidance from marshallers.
Under no circumstances may aircraft self
park without guidance from marshallers’.
GA Apron: Pilots will be
directed by ATC to taxi to the Jet Centre. Pilots must then follow the
marshaller’s instructions for parking.
To ensure adequate wing tip
clearance is maintained from adjacent stands, it is imperative that
pilots follow the lead-out markings on all stands.
If the aircraft is not adequately positioned on stand the pilot should
proceed as directed by ATC. Prior to undertaking the manoeuvre, in
order to re-position onto stand, the pilot must request permission
from ATC to enter the taxilane and advise ATC if the aircraft is
unable to follow the stand lead-off line. Only once permission has
been granted by ATC shall the pilot commence the movement into the
taxilane.

Just shows that space is really tight. So to complete the  answer to your question, sometimes the need for a certain disposition and use of the area is not just for maximising the number of parking slots, but it could have been a larger picture imperative - done to enable the implementation of the project at all in the limited area  and disposition of usable land between the main road and river.


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Why did most airliners have black noses in the 60's and 70's?
Here are a couple of examples:

This is a guess but I'd suspect it has something to do with radar.

I think this question could be enhanced with some recent photos of newer planes that do not feature such a black nose, which undeniably exist. There are already some answers now that just state some planes now also have black radar domes, which is certainly true, but does not address your perceived transition from "most airliners have black noses" to most airliners having their noses coloured the same as the rest of the hull.

Under the nose of modern airliners is weather radar that helps the crew to see and avoid hazardous weather conditions. This is required by FAA CFR §121.357 for transport category aircraft. The radar signal would be blocked by the aluminum that makes up the rest of the fuselage, so the radome is made of a non-metallic material, generally a fiberglass composite.
These threads discuss the topic. The last post in the first linked thread suggests that most paint needed lead to prevent fading and/or a zinc-based primer, which would interfere with the radar. Black was one color that did not require these. Modern paint has done away with these metal additives, allowing more color options without interfering with the radar.
The Museum of Flight claims that the black color is only to remind crews to not paint over the radome. Painting over it would affect the radar signals.
According to this thread, the color of the radome affects the operation of the weather radar underneath. They were painted black because it has the lowest reflectivity. Modern radars are able to deal with different colors much better.

Your last paragraph is extremely dubious. I'd guess that because radar-transparent paints were developed, probably using titanium instead of lead, that it no longer was an issue. That different colors matter is tenuous; the hard part is getting a very bright, opaque, white base to the paint.

@NickT thanks for the tip, I included a couple of better references to that point.

Because the nose of the plane is where the weather radar antenna is, and paint would interfere with the signals.
This is not just the 60's and 70's, and its not only on the nose. Here's a brand new PC-12 with a radar dome (radome) on the right wing, painted black:

Inside of the radome is a radar array that can be tilted and panned using a control inside the cockpit:

This is the pilot's view inside the cockpit, which shows the knobs used to adjust the pan/tilt, range, and sensitivity:

And you can read a brochure about a particular aviation radar system here.

"Brand new PC-12"? Look at all those interior scratches!

that interior is not from the PC-12, anymore than the nose cone radar array is.

Is it just me or does the last image look like it was taken from a video game? Look at all those screws completely lacking any depth at all.

@Chris It looks like it's from 727 Captain.

@rbp Isn't the antenna in your middle image a phased array antenna? Isn't the point of such that it can transmit in different directions (and receive the echo from that direction) without being physically rotated? But I guess there is a low limit to the deflection that can be achieved, and sometimes one would want to check the weather further up/down/sideways, too. Anyway, back in the 60s and 70s, that the question is about, there were no phased array radars, right? The weather radar in aircraft had a traditional antenna continuously moving from side to side (and up and down), right?

It is called a nose cone.
There can a several reasons (as mentioned here and here):

Aircraft used to be painted with a lead-based paint, which is unsuitable for radomes. Instead, they had a rubber coating. Nowadays,
  the paint used (epoxy on military planes, I believe) won't affect the
  radar.
It's as much a fashion statement as anything else
It started out as an anti-glare panel. The fuselage within pilot's view is painted dark and of matte finish to reduce the chance of any
  glare that might obscure pilot's vision.

Even today, you can find some military aircraft with a darker nose cone.

This RC-12K is a great example of a military aircraft with a black nosecone and anti-glare on the inner engine cowlings and in front of the windscreen.


text:
Why don't airlines load their passengers like freight?
Apparently, the time for boarding and deboarding an airplane is a large cost factor and something airlines try to minimize but have trouble controlling because the passengers do it themselves.
A solution would be to have passengers board a passenger cabin outside of the airplane and then slide that cabin into the plane via a nose-hatch, much like large volume freight. Has this been considered? If yes, why was it not pursued?

Do you really want to treat passengers like cattle?

Why don't airlines load their passengers like freight?  You obviously haven't flown on some of the airlines I have,

Where would said cabin(s) physically be located? The logistical expense would seem just as large. Also more time without a lavatory.

They are called "self-loading cargo" for a reason. Your solution of a flatbed Jumbo would make pressurization harder and reduce payload substantially.

Passengers tend to object to being packed into cargo bins and left sitting upside down by inattentive ground handlers...

All the mentioned issues aside, how would you suggest to implement emergency exits in such a scenario?

Or physical access between cockpit and cabin? (Yes, it's sometimes needed! If for no other reason then because the pilots probably want to get in there somehow so they can do their job. You aren't proposing that they should be climbing in through the side windows, are you? :-))

When I read your Title, I thought to myself, "He couldn't really expect us to be loaded onto a plane by a Forklift, could he?" But then I read your question, and it sounds a little better now.

So you want people packed in like sardines even SOONER? Have you ever flown with kids? This sounds like a nightmare!

@corsiKa Those kids would be much less of a nightmare if jammed tightly together and packed in a tomato sauce.

@Simon, I signed up to A.SE just to upvote your comment.

I'd be very willing to travel in a 'capsule hotel' type unit if it were the same price as an economy seat, because I'd get to lie down and go to sleep. I wouldn't at all mind it being stacked above/below other capsules. I would need to get out occasionally to go to the toilet though.

You can make it more efficient by dispensing with the cabins altogether!

Boarding takes a long time, true. There are more efficient strategies (.pdf) which have been proven to speed up boarding by between 20% and 50%, still they are not adopted. Why?
You need to load the baggage, too. Also the catering. And fuel. You need to inspect the aircraft, run through checklists and brief the new crew. This all takes up time, too. Airlines do not see a big saving by loading passengers faster, and then waiting for all other jobs to be completed.
Your radical idea of containerizing passenger transport would certainly speed up things, but it would cost a lot of payload.

The most efficient pressure vessel is a sphere, and the next efficient is a cylinder with spherical ends. Pressurizing a boxy container is structurally very inefficient.
Loading passengers on palettes first and sliding those palettes through a narrow opening takes time in itself. And the palettes have their own weight, their own floor, their own support structure for the baggage racks, their own cabeling for the entertainment system. They would, however, not need their independent pressurization, so their weight impact would be moderate.
A possible "flatbed" airliner which could attain the shortest boarding times has only half as much fuselage height (or less) to transmit bending loads along its length. See here for an example of buckling in a fuselage skin due to bending loads. Now consider that reducing height by 50% will need four times more cross section in the longerons to keep stiffness constant. That will make the fuselage much heavier.

In the end, airlines go with the solution which maximizes their profits. By using a hollow cylinder and let their "cargo" self-load, they can maximize their payload and get tolerable boarding times. Tolerable for them, that is. That better loading procedures could allow you to come to the airport later is of no interest to them. If they would see an advantage in shorter boarding times, there would be ample of opportunity by improving strategies, even with existing airplane designs.

I don't think it's certain that it would speed anything up. Loading a fuselage-sized container into a plane would be a slow, delicate job and I can imagine it taking rather longer than just getting 300 people through a door.

@DavidRicherby: When you have standard containers and let the aircraft taxi below two gantry cranes, one for taking the arriving container off and one for putting the departing container in place, the aircraft would be ready in a jiffy. I am sure by doing it cleverly that time can be saved.

@PeterKämpf this solution does not solve the problem of passenger who choose to do last minute shopping and arriving late at gate. I did that myself. It is more of a human factor that is limiting factor here.

@HorusKol The point isn't to speed up the loading time from the passenger's perspective, it's to speed it up from the aircraft's perspective. Aircraft are big expensive assets so the less "dead" time they have on the ground the better.  Speeding up turnaround times in general is a big win.

@PeterKämpf The question talks about sliding a capsule into the fuselage through a nose door, rather than piggybacking it on top of something. But I agree that the piggyback solution would give much faster loading.

@DavidRicherby: You're right - the "hatch" term got me confused. And a Boeing concept from the Eighties of a "flatbed Jumbo". But the point is the same, nevertheless: The airlines don't see a need to speed up boarding.

"allow you to come to the airport later is of no interest to them" - that may depend on the distance travelled. The extra time spent at the airport has negative impact on being competitive with trains or cars over relatively short distances (though this lack of competitiveness may be seen as a global advantage with respect to ecology)

Let's face it. We humans, as evolved as we are, are still incapable accomplishing the simplest of tasks. Maybe you should have a big sign that reads "FREE FOOD AND DRINKS FOR THE NEXT 5 MINUTES ONLY!" at the other side of the entrance. I bet they'll move much quicker...

I had never heard of the Steffen method until a day or two ago, but I've always thought that's how planes should be loaded. It seems obvious that it's the optimal solution. It doesn't allow for priority boarding, though, which may be part of why it isn't done in practice. Those people up front always need to board first so everyone else on the plane can bang their baggage into them as they walk down the aisle, I guess. The Steffen method sure would be nice on very cold or very hot days rather than having everyone stand in the jetway because someone in row 3 is trying to shove in a bag.

@Peter Kämpf I wasn't necessarily thinking of loading a container, but rather a tray (i.e. only a floor with mounted seats and baggage holders) and sliding it into a slit in the inside of the -pressurized- fuselage, like into a tray holder.

@SebastianHenckel: Yes, I first got that wrong. What takes most time during boarding is people stuffing their oversized baggage into the overhead racks, blocking the aisle for everyone else. By sliding in trays with only seats, the stuffing part still needs to happen, with all the shuffling and waiting it involves. Wouldn't it be best to have that located outside the airplane? Then a fully seated compartment with all the carry-on baggage neatly stowed would be loaded in one go. That was my idea when I read the question first.

@HorusKol to be honest it didn't occur to me that anyone might think the question didn't contemplate having the "capsule" pre-loaded before the aircraft landed, since otherwise it would make no sense whatsoever.

the next best shape would be a cylinder with spherical ends I thought it would be an egg

The system you propose would be extremely heavy compared to existing aircraft. More weight means more fuel and more fuel means even more fuel to carry that fuel through the air, and even more fuel to carry that fuel and so on.  Fuel is one of the biggest expenses an airline faces.
It would also be extremely expensive.  To gain any time, you'd have to have the departing passenger capsule loaded and ready before the arriving passengers have left their capsule. That means you need twice as many capsules as planes.
And the time saving wouldn't be all that great.  You'd still have to refuel the plane and load and unload the cargo.  Manoeuvring a fuselage-sized capsule into the plane would be a long job.  I'm far from convinced that it would be faster than getting however many hundred people through the door.

More to the point, maneuvering a fuselage-sized capsule full of passengers without jostling or injuring any of the occupants.

@Shadur If you were fully committed to the passengers-as-cargon idea, you wouldn't worry about jostling the cargo. ;-)

@Shadur -- well, don't forget, they have to do that with the plane as a whole.

The first and biggest concern would be the weight of the container. They need to have walls, floor and a ceiling and be able to lock in place on the plane.
Also each division between containers would remove some room available for another row of seats in the traditional method.

Don't forget that the most efficient geometry for a pressure vessel is a sphere, followed by a cylinder with spherical ends. The boxy container is impossible to pressurize without substantial deformation.

@PeterKämpf which means that the pressurization must come from the fuselage still, I didn't forget that. It also means that the "nose-hatch" much be strong enough to withstand pressurization.

@ratchetfreak The nose hatch withstanding pressurization isn't a particularly intractable problem, though. Such aircraft already exist. I don't feel like doing the calculations right now, but it wouldn't surprise me at all if the pressure applied to the outside actually exceeds the pressure being applied from the inside on the front section of the aircraft (which is, obviously, facing the airstream.)

Passenger crates were envisaged a long time ago. Apart from the above objections, may I add this: A passenger crate in a plane is a box in a box. This would worsen claustrophobia which is already a problem. A view outside would alleviate this, but requires windows. Crate windows would need to be aligned with fuselage portholes.
The psychological problem is doubled by a physical problem which is emergency evacuation.
Any project would need a highly standardised crate compatible with competing aircraft types. This standard would later become a constraint for future design innovation. It would also limit customisation of planes by constructors for specific customers.

Emergency evacuation is a very good point (eject the capsule!!! ;-). But I disagree on standardization, as I don't see why you'd need a standardized crate to get the benefit of a faster turnaround for the main plane. And surely it would increase customization? Most customization these days is in cabin layout but, in this scheme, the cabin is a completely separate module.

@DavidRicherby: To load passenger crates independent of aircraft type and position within the plane will need a lot of standardization: Everything needs to line up correctly, from air ducts to escape doors. Paul has a very valid point here.

@PeterKämpf Yes, to do it independent of plane type would require standardization. But why would it need to be independent of plane type? For example, airports today cope just fine with loading Boeings and Airbuses of various different sizes. Why couldn't they load 777 capsules into 777s, A380 capsules into A380s, etc.? The problem seems to be with the capsule concept, not with different planes taking different capsules.

I wouldn't want all of the connections for pressurisation, air conditioning, lighting, water, IFE etc etc connected and disconncted every flight.  A maintenance and safety headache.

Something even more radical has been considered! Airbus filed a patent on replacing the entire upper fuselage - seats, passengers and all - during a layover. The structural problems inherent in that idea look pretty hard though.

This doesn't really answer the question.

"Has this been considered? If yes, why was it not pursued?" Okay, so it slides in from the top/side rather than through the nose, but it's the same idea.

I for one would jump at the chance to arrive earlier, "board" earlier and sit in cramped airline seating even longer so the airline can save money.
If you didn't pre-load the passengers in your capsules before the plane was ready to depart, there would be no time savings.
No thanks. It's bad enough they hold you prisoner on the plane when there are hour + long delays in departure.

That has nothing to do with boarding and everything to do with how pilots and air crew are paid. Basically less or not at all when the plane is hooked into up at the terminal. Pay starts when the door closes.

My suspicion is that delays in loading passengers onto aeroplanes come from two sources:
1) Security & border concerns since the hijacking of aircraft became common in the 1970s;
2) Airports' desire to make money from passengers being forced to wait in airports, where they buy things from retailers who are paying tenants of the airport.
On the topic of (2) a British airport group (Heathrow is one of their properties) told shareholders around 15 years ago that they were not in the transport business, but in the retail business, pointing out that over 60% of their revenues came from rents paid by airport retailers.
It's elementary economics that while this is the case, airports and airlines have no incentive to make aircraft boarding quicker, even if a solution for (1) were found. Were airports to shift their revenue model away from retail, this entails airlines paying more for landing slots, so the two groups have a common interest in wasting the time of travellers by keeping them in airports for as long as possible out of their travel time.
A saner approach to speeding up the loading of passengers would be - rather than loading them in boxes like freight - to simply cut the airport out and have passengers board special buses in town, check their bags and travel documents with a couple of officials who are also travelling on the bus, and have those buses drive straight onto the tarmac and up to the aeroplane.
This is after all how very rich people travel. Someone drives them straight to the aeroplane.
Airports earning 60% of revenues from in-airport retail tenants are unlikely to support this simple idea, however.

This answer seems to be subjective with personal opinions. I also highly doubt if your "bus direct to terminal" solution is practical at all.

I don't think that this is what the question is about. Time lost in the airport does not cost the airline anything, it's the time spent by the aircraft on the ground that is a large cost factor. Your “solution” would actually be worse, from the airline's point of view.

I understand your point, Relaxed, that the question seeks to reduce aircraft loading times, though I'm not sure why bus-loading would be worse. Is it subjective, kevin, to consider who benefits commercially from the current status quo? And why would travellers loaded into a kind of pallet favour airports, since pallets could be transported from anywhere? I'd argue that the bus-to-the-aircraft solution must be more practical: planes can park on a much larger area of tarmac away from buildings. Some airports take passengers to the plane in buses already, so why not drive buses out of town?

@MarkGriffith For one thing, busses in town are accessible by millions of people with no security controls at all. Therefore, to maintain the current level of security, they'd all have to be searched thoroughly before being allowed to enter the ramp. Keeping the entire aircraft in a secure area and just checking the bags and passengers is much more cost and time efficient.

Also, if airliners boarded faster, passengers would, on average, spend more time in the terminal where they could be buying stuff, not less. Passengers can't be buying stuff while they're standing in line in the jetway waiting on the guy in row 3 to shove his bag in the overhead. If boarding took only 10 minutes instead of 20, for example, that's 10 minutes longer people can wait in the terminal before boarding where they could be buying stuff.


text:
What is the typical temperature of an airliner's hull during flight?
Some high-speed military aircraft like the SR-71 had real heating problems, but airliners also travel almost at the speed of sound, use most of their fuel to make up for frictional losses, so I would presume that their hulls heat up. They are also cooled by the airflow, but at what temperature does equilibrium set in during cruise? I remember that airliners don't seem to be very hot when you touch them after landing, but they have had time to cool down in the slow winds during descent.

use most of their fuel to make up for frictional losses - what gave you this idea?  Since efficiency is < 50%, most of the fuel is simply burnt for no return.  The majority of what is left is burnt to overcome drag which is a consequence of creating lift.  I don't have a number, but the overall amount of fuel used to overcome friction is going to be a small fraction.

There are two primary factors that affect the skin temperature of an aircraft in flight: the air temperature, and the speed of the aircraft.
The air temperature where airliners cruise is relatively cold, around -54 °C at 35,000 feet.
As a body like an aircraft moves through air, it compresses the air, which causes the air temperature to rise. The maximum temperature rise will be if the air is completely stopped, such as at a leading edge. This is called the total air temperature, and the amount that the temperature rises is called the ram rise.
Using a simple formula to find the ram rise:
$$RR = \frac{V^2}{87^2}$$
… where $RR$ is in Kelvin, and $V$ is the true airspeed in knots.
Using a typical airliner cruising speed of 500 knots gives a temperature of 33 degrees. This brings the total air temperature to -22 °C, which is still quite cold. At places other than the leading edge, the temperature rise will be less. This is why cargo holds will need heaters to be safe for live animals, even being insulated and pressurized. Airliners just don't fly fast enough to produce a significant amount of heating.
On the other hand, the SR-71 could fly at over 1,910 kts, which gives a ram rise of 482 °C. The air doesn't get much colder as you climb to the altitudes where the SR-71 flew, so this gives a total air temperature of over 400 °C. Speed makes a huge difference.

Where does the 87 come from in that equation?

@Holloway If you follow the linked reference you'll see it is an empirical approximation (for the specific system we're discussing) lumping in the heat capacity and recovery factors in the analytical equation.  The = should probably be an ≈ in this instance.

@J... Thanks, I imagined it was a mix of constants but wasn't sure which.

alternately, you can use the isentropic flow relations and get $T \approx T_\infty(1+\frac{v^2}{531.6\cdot T_\infty})$, where $T_\infty$ is the air temperature in Kelvins and $v$ is the speed in knots

This answer calculates temperature rise due to compression, however it does not address the friction between the air and the surface of the aircraft. I'm assuming the frictional heating is negligible, but someone might want to elaborate on it.

So would the lower pressure at the lifting surfaces of the wing cause the air temperature to drop below ambient temp?

@TomMcW yes, the lower pressure will also lower the air temperature there.

I think there is a confusion degré-celsius/kelvin because 33 kelvins =-240°C

@L'aviateur : The difference of 33K is the same as the difference of 33°C. If you wanted to have everything in Kelvin then you should also convert -54°C to 219K, then 219K+33K=252K or -21°C. It works the same if you just add 33°C: -54°C+33°C=-21°C.
Sorry for necro of a necro.

Local air temperature
In fast aircraft, the maximum heating is at the stagnation point. Here the kinetic energy of the flow is completely converted into pressure, which heats the air and, consequently, the structure. Due to the low local speed and the high pressure at and near the stagnation point, the rate of heat transfer is high, too, adding to the heat load.
The formula for the stagnation point temperature $T_s$ of an ideal gas of the temperature $T_{\infty}$ hitting an object with the Mach number Ma is $$T_s = T_{\infty} + T_{\infty}\cdot\frac{(\kappa-1)\cdot Ma^2}{2}$$
For air the ratio of specific heats $\kappa$ is 1.4. The tip of the fuselage nose of an airliner flying at Mach 0.85 will see air temperature to rise by 14.45%. If the air at altitude has a temperature of 220°K (-53.15°C), the air temperature at the stagnation point will be 251.8°K (-21.36°C).
But past the stagnation point the air will accelerate and become faster than flight speed. Now pressure and, consequently, temperature need to drop sufficiently to encourage the flow to stay attached and follow the curvature of the forward fuselage. This acceleration will cool the air, so the flow right above the windshield will be cooler than the ambient air.
Along the cylindrical portion of the fuselage, we find roughly flight speed again, but now friction will change the temperature close to the wall. Again the kinetic energy is converted, but the heating is caused by friction. See the boundary layer plots below:

Frictional and thermal boundary layer (picture source)
The temperature close to the wall is now called recovery temperature and is different from the stagnation point temperature because there is a small speed component normal to the surface which carries away some of the heat. The air temperature depends on the ratio between viscous diffusion and thermal diffusion, which is expressed by the Prandtl number Pr. If Pr>1, the air temperature at the wall is higher than the stagnation temperature and for Pr<1, it is colder. The Prandtl number of air is 0.72, so the air surrounding the fuselage is slightly colder than the stagnation temperature.
Fuselage temperature
The fuselage temperature is determined by the equilibrium between thermal conductivity, radiation and convection.

Conductivity: Here it is important how much the internal temperature of the fuselage can heat the skin. Cabin temperature is likely around 20°C, so some heating can be expected. However, since most airliners have isolation mats between the outer skin and the internal wall panels, conductivity from the inside is not dominant and will likely raise the skin temperature by a few degrees or less. The low heat conductivity of air (0.0204 W per m² and Kelvin) means that the heating from the inside dominates conductivity.
Radiation: Since the top of the fuselage is pointing into space, its far-field radiation budget is negative at night and where it points away from the sun, so radiation will cool it. The lower fuselage, however, is facing either the ground or clouds below, which both are likely hotter than the ambient air. Radiation will not cool it much and is more likely to heat it up. The part of the fuselage in direct sunlight will be significantly hotter again, depending on its color.
Convection: This is the dominant factor due to the high speed of the air around the fuselage. Here the air and the fuselage exchange heat by near-field radiation, and since the layer of air is replenished quickly and continuously, the air temperature is impressed on the fuselage.

I did not go to the effort of calculating the end result, but tried to list the main contributors and their magnitude. In general, the fuselage temperature is slightly below the stagnation temperature, and a dark fuselage in bright sunlight or one with little insulation and a hot interior will be several degrees hotter than the stagnation temperature.

You distinguish between the kinetic energy of the flow converted into pressure at the stagnation point and 'friction' caused by the airflow against the hull - isn't the first friction too? In both cases, the body of the aircraft transforms the uniform speed of air  molecules into heat.

@yippy_yay: No, in the first case it is the pressure rise which heats the flow reversibly, and friction heating is irreversible and isobaric.

Okay, but once the heat inside the stagnation point flows into the hull, the process is irreversible. You would still distinguish between friction and this process?

@yippy_yay: Yes, because there is no friction involved. Compression heat occurs in an ideal gas, too.


text:
What are the regulatory categories of aircraft defined by the FAA?
What are the different aircraft regulatory categories aircraft defined by the FAA? Are there any exceptions?

The FAA defines several different types of aircraft, which have different applicable regulations. For fixed-wing aircraft, airworthiness for normal, utility, acrobatic, and commuter categories is covered in Part 23.
Note: Seat number excludes pilot seats. Weights are maximum certificated takeoff weights.

Normal category

Seats: 9 or less
Weight: 12,500 lb or less
Allowed maneuvers:

Any maneuver incident to normal flying;
Stalls (except whip stalls); and
Lazy eights, chandelles, and steep turns, angle of bank 60 degrees or less

Utility category

Seats: 9 or less
Weight: 12,500 lb or less
Allowed maneuvers:

Normal category, plus:
Spins (if approved for the particular type of airplane); and
Lazy eights, chandelles, and steep turns, or similar maneuvers, angle of bank can be more than 60 degrees but not more than 90 degrees

Acrobatic category

Seats: 9 or less
Weight: 12,500 lb or less
Allowed maneuvers:

Anything not prohibited due to flight tests

Commuter category

Seats: 19 or less
Weight: 19,000 lb or less
Engines: 2 or more
Allowed maneuvers:

Any maneuver incident to normal flying;
Stalls (except whip stalls); and
Steep turns, angle of bank 60 degrees or less

Except for commuter category, airplanes may be type certificated in more than one category if the requirements of each requested category are met.

Beyond commuter category, there is the transport category. Multi-engine airplanes satisfying either of the below requirements must be certified in this category. Airworthiness for the transport category is covered in Part 25.

Transport category

Seats: 20 or more
Weight: more than 19,000 lb

For rotorcraft, there are two main categories: normal and transport. Airworthiness for the normal category is covered in Part 27.

Normal category

Seats: 9 or less
Weight: 7,000 lb or less

Multi-engine rotorcraft satisfying these conditions may be certified under transport Category A (see below) if they satisfy certain conditions.
Airworthiness for the transport category is covered in Part 29. There are sub-categories here as well.

Transport category

Category A

Seats: 10 or more
Weight: more than 20,000 lb

Category B

Seats: 9 or less
Weight: 20,000 lb or less

If a rotorcraft only satisfies one of the requirements for Category A, it may be certificated under Category B with certain additional requirements.

A multiengine rotorcraft may be type certificated as both Category A and Category B with appropriate and different operating limitations for each category.

There is also a manned free balloon category covered in Part 31.

A balloon is a lighter-than-air aircraft that is not engine driven, and that sustains flight through the use of either gas buoyancy or an airborne heater.

Part 21 addresses the certification process itself, and includes special types of certificates for other aircraft.

Primary category

Power sources allowed:

None
Single naturally aspirated engine (stall speed 61 kt or less)
Rotorcraft with 6 lb per square foot main rotor discs loading limitation (sea level standard day)

Weight: 2,700 lb or less (seaplanes 3,375 or less)
Seats: 3 or less
Unpressurized

The restricted category allows a plane to not meet certain requirements under the normal regulations. The aircraft must meet one of the following conditions:

Restricted category

Requirements of any regular category, except any that are "inappropriate" for the aircraft's purpose
Armed Forces aircraft modified for a special purpose
The certificate is "restricted" to some specific purpose such as water bombers or crop-dusting and does not allow use in general commerce.

US Armed Forces surplus aircraft can also be certified under one of the below conditions. The FAA has some leeway to waive requirements or add additional ones.

Surplus aircraft of the Armed Forces, also known as Limited category

Designed and constructed in the US, satisfies certain regulations
Counterpart of previously certified civil aircraft, complies with those regulations

Special certificates are issued for cases where the aircraft is not certified under the regular categories. These allow operation of certain aircraft for certain purposes.
The experimental certificate is for special cases.

Experimental certificates

Research and development
Showing compliance with regulations
Crew training
Exhibition
Air racing
Market surveys
Operating amateur-built aircraft
Operating primary kit-built aircraft
Operating light-sport aircraft

The light sport certificate is for light aircraft that may be operated under special conditions. There are 5 categories of LSA.

Light sport certificates

Airplanes
Gliders
Powered parachutes
Weight-shift-control aircraft (commonly called trikes)
Lighter-than-air aircraft (balloons and airships)

This answer was correct when written but it is now out of date, the regulations have had a major revision.

A large portion of part 23 was changed at the end of 2016. The utility, acrobatic, and commuter categories will not be used for new certifications. Old aircraft will retain the old category limits used for their certification.
In place of the old system the "normal" category now covers up to 19 passengers and 19000 pound gross weight, it is divided into eight risk levels based on number of seats and low or high speed.
Aerobatic[not the old acrobatic category] is a performance certification not a general category, with performance limits specified in the POH on a model by model basis.
The new system does not prescribe methods to meet the desired risk level it only describes the desired result so that plane makers are free to invent new methods. The old methods are still considered valid for meeting the new standards.
I am still grocking the new part 23, but it seems that maneuvering limits will be left to the manufacturer and each aircraft POH will specify its own list of limitations. The manufacturer must show data for expected operating conditions and how the design meets the needs of those conditions.
There are many more changes, notice that the new part 23 uses section numbering of the 23.2xxx format while the retired section 23 used 23.xxx
Title 14 CFR
§ 23.2005 Certification of normal category airplanes.
(a) Certification in the normal category applies to airplanes with a passenger-seating configuration of 19 or less and a maximum certificated takeoff weight of 19,000 pounds or less.
(b)Airplane certification levels are:
(1) Level 1 - for airplanes with a maximum seating configuration of 0 to 1 passengers.
(2) Level 2 - for airplanes with a maximum seating configuration of 2 to 6 passengers.
(3) Level 3 - for airplanes with a maximum seating configuration of 7 to 9 passengers.
(4) Level 4 - for airplanes with a maximum seating configuration of 10 to 19 passengers.
(c)Airplane performance levels are:
(1) Low speed - for airplanes with a VNO and VMO ≤ 250 Knots Calibrated Airspeed (KCAS) and a MMO ≤ 0.6.
(2) High speed - for airplanes with a VNO or VMO > 250 KCAS or a MMO > 0.6.
(d)Airplanes not certified for aerobatics may be used to perform any maneuver incident to normal flying, including -
(1) Stalls (except whip stalls); and
(2) Lazy eights, chandelles, and steep turns, in which the angle of bank is not more than 60 degrees.
(e)Airplanes certified for aerobatics may be used to perform maneuvers without limitations, other than those limitations established under subpart G of this part.

Why'd they change it, and what category would unmanned IFR-certified aircraft fall under?

@Sean The intent is to make the rules less prescriptive and allow disruptive technologies (e.g. electric aircraft, FBW) to be used on Part 23 without stacks of equivalent safeties. In reality, the old rules are now written verbatim into ASTM as accepted means of compliance.


text:
Who decides when landing gear requires an overhaul?
"If the landing gear on your aircraft is required to be overhauled periodically..."
So is there a case where its been decided that landing gear does not require to be overhauled?

Yes.  Most light general aviation planes do not require gear overhauls.  Most (all?) turboprops and jets do.  The manufacturer (regs?) will specify intervals.

The aircraft manufacturer in consultation with the regulatory authority will publish a maintenance schedule that details what maintenance is required and at what interval.
For simple types the maintenance my be no more that a visual inspection and a tyre pressure check.

I am sure the  Navy  has serious inspection requirements as  high strength steel landing gear are subject to hydrogen stress cracking ( aka , hydrogen embrittlement ) in corrosive environments such as sea coasts.

Thanks for your reply. For commercial aircrafts, is there a regulation that requires for landing gear to be overhauled.

There's unlikely to be a specific regulation that says 'overhaul the undercarriage'. There is a regulation that says that operators must follow the manufacturer's maintenance schedule. If that calls for an undercarriage overhaul then it must be done. Exactly what that entails will vary by type and maintenance interval.

@user46429 There is no regulation directly stating that landing gear just be overhauled, only that parts on an aircraft must be airworthy, which a gear past it's TBO isn't.


text:
Does getting a type rating count as a flight review?
I have a question for BFR and Flight Currency.
Currently, I'm going to work for a small-sized local company as a First Officer.
and my last check-ride for COMM-MEAD (Commercial Multi Add-on) was 8th, AUG, 2019.
As far as I know, I don't need 'Flight Review' for at least 24 calendar months. (In other words, my flying can be conducted legally 31st AUG, 2021.)
and the training plan, which issued by our company, says we'll get a new type-rating for ERJ-145 on MID of JULY, 2021.
(To be exact, I don't have type-rating for ERJ-145 yet. (but training has been completed. Only type-rating, that's the one of the missed item what I don't have yet.))
So I suppose, I don't need to take a 'Flight Review', because according to schedule, we'll get a new type-rating on MID of JULY, 2021.
Then After I get a new type-rating for ERJ-145 on MID of JULY, 2021, 'New period' for 'New 24 calendar month' will start from MID of JULY, 2021. so it will be valid for at least the END of JULY, 2023.
but the 'Company' says, still I need to do a 'Flight Review'.
Is there any one explain it clearly for me? Do I need to do a 'Flight Review'?
PS. the new type-rating for ERJ-145 will be a SIC privilege only.

So the question is whether getting your SIC type rating counts as a “checkride” for BFR purposes?

CFR 61.56 states:

Flight review.
(a) Except as provided in paragraphs (b) and
(f) of this section, a flight review consists of a minimum of 1 hour
of flight training and 1 hour of ground training.
(d) A person who has, within the period specified in paragraph (c) of
this section, passed any of the following need not accomplish the
flight review required by this section:
(1) A pilot proficiency check or practical test conducted by an
examiner, an approved pilot check airman, or a U.S. Armed Force, for a
pilot certificate, rating, or operating privilege.

Typically Part 135 and 121 operators conduct annual proficiency checks of their pilots.  Reference CFR 135.293 for an example of what is included in the requirements for a part 135 certificate holder to conduct a competency check of their pilots.
It should be noted that it references the generic term "pilot" as opposed to distinguishing between PIC and SIC.  You will also note that these requirements are very similar to what is needed for a 24 month flight review.  The FAA recognizes this, which is why 61.56 says that you don't need a 24 month review from a CFI if you have had a 12 month pilot competency check from a qualified check airman.
Therefore, per 61.56 you should be able to count 12 pilot competency check as your 24 month flight review for any personal flying you might want to do.
If you are asking us whether your company's check flight for the type rating also counts as their annual proficiency review, you will need ask your company directly.

The term is pilot proficiency check.  I hope I'm not pulled over for running a light.   "Officer", I say, "the light was green!".  The officer responds, "the regulation simply uses a generic term "light" as opposed to "red or green light".   Somehow, I think the words in the regulation matter.  Just saying.

Also note the term used is "pilot", not PIC or SIC...

§61.56 Flight Review

(a) Except as provided in paragraphs (b) and (f) of this section, a flight review consists of a minimum of 1 hour of flight training and 1 hour of ground training. The review must include:

(b) Glider pilots may substitute a minimum of three instructional flights in a glider, each of which includes a flight to traffic pattern altitude, in lieu of the 1 hour of flight training required in paragraph (a) of this section.

(c) Except as provided in paragraphs (d), (e), and (g) of this section, no person may act as pilot in command of an aircraft unless, since the beginning of the 24th calendar month before the month in which that pilot acts as pilot in command, that person has—

(1) Accomplished a flight review given in an aircraft for which that pilot is rated by an authorized instructor and

(2) A logbook endorsed from an authorized instructor who gave the review certifying that the person has satisfactorily completed the review.

(d) A person who has, within the period specified in paragraph (c) of this section, passed any of the following need not accomplish the flight review required by this section:

(1) A pilot proficiency check or practical test conducted by an examiner, an approved pilot check airman, or a U.S. Armed Force, for a pilot certificate, rating, or operating privilege.

(2) A practical test conducted by an examiner for the issuance of a flight instructor certificate, an additional rating on a flight instructor certificate, renewal of a flight instructor certificate, or reinstatement of a flight instructor certificate.

§135.293   Initial and recurrent pilot testing requirements.

(b) No certificate holder may use a pilot, nor may any person serve as a pilot, in any aircraft unless, since the beginning of the 12th calendar month before that service, that pilot has passed a competency check given by the Administrator or an authorized check pilot...

§135.297   Pilot in command: Instrument proficiency check requirements.

(a) No certificate holder may use a pilot, nor may any person serve, as a pilot in command of an aircraft under IFR unless, since the beginning of the 6th calendar month before that service, that pilot has passed an instrument proficiency check under this section administered by the Administrator or an authorized check pilot.

Flight reviews are for PIC rated pilots as stated in 61.56(c).  First officers do not need a current flight review to provide SIC duties on airplanes they are qualified for under 61.55.  For 135 operations, they need a competency check every 12 months.
A competency check (135.293) is different than a proficiency check and it does not constitute as a substitute for a flight review under 61.56(d)(1).
An instrument proficiency check (135.297) does count as a flight review under 61.56(d)(1).
The checkride for a SIC and PIC under 135 are different.  If you are curious, you can find the minimum requirements for a SIC check in FAA order 8900.

Update to answer comments
Definition of competence

adequacy; possession of required skill, knowledge, qualification, or capacity Dictionary.com

the ability to do something successfully or efficiently. Bing

Definition of proficiency

the state of being proficient; skill; expertness: Dictionary

a high degree of competence or skill; expertise. Bing

The FARs do not define these terms and so common definitions are used.  As of June 2021, there are 10 references to competency check and 45 references to proficiency check.  There are no legal interpretations concerning flight reviews and competency checks or proficiency checks.
If the terms "competency" and "proficiency" are synonymous as suggested in the comments, there would be no reason for this verbiage in 61.55.  The authors would have simply stated one term for 91k, 121, 125 and 135 instead or purposely separating out the parts for each check.
61.55 Second-in-command qualifications

(j) When an applicant for an initial second-in-command qualification for a particular type of aircraft receives all the training in a flight simulator, that applicant must satisfactorily complete one takeoff and one landing in an aircraft of the same type for which the qualification is sought. This requirement does not apply to an applicant who completes a proficiency check under part 121 or competency check under subpart K, part 91, part 125, or part 135 for the particular type of aircraft.

The only time a SIC and proficiency check are together in the FARs in is Part 121 and I am curious if that change was due to the 1,500 hr rule for airline pilot hiring.
All the other proficiency checks in the other parts are for pilot in commands.  Competency checks are used for privileges as in 61.31 (e) and 61.31(f) as well a SIC competency checks for 91K, 125, and 135..
61.157(f). Flight Proficiency for ATP candidates a candidate can receive an ATP certificate after completing both a competency check and a proficiency check.  Again proficiency for pilot in commands.

All of this logically leads to one conclusion.  The authors of the FARs purposely differentiated between the terms competency and proficiency.  They also purposely left out the term "competency" as a exception to the flight review requirements in 61.55(d)(1).
To further prove the point.  A practical test for a certificate or rating requires both proficiency and competency.  §61.43(a)(3).
Competency is a minimum standard to do a job.  Proficiency is a much higher standard.  I would prefer my doctor to be proficient instead of competent before he operates on me.  Just saying.

This sounds like a good answer that might trump mine, but would you please post a  quote, or page and paragraph reference that you are interpreting? (It is a big order.)  I understand that part 135 and 121 PIC and SIC checkride requirements differ, but I don't think the boilerplate language "act as PIC" (on other aircraft that you are NOT limited to SIC...) is in conflict with my interpretation.  Because 61.56(d)(1) uses the generic term "pilot" in regards to the proficiency check received as opposed to PIC or SIC.  If they intended to exclude SIC checks they would surely have been specific.

@MichaelHall.  That is a great question.  See my updated answer.

You haven't really expanded on why you are interpreting the regs this way:  "A competency check (135.293) is different than a proficiency check and it does not constitute as a substitute for a flight review under 61.56(d)(1)."  This is actually the exact opposite of what 61.56(d)(1) says!

Where in 61.56(d)(1) does it use the wording competency check?  If a proficiency check and a competency check are the same, why does the regulations differentiate them?

Good point. You are making a pretty conservative interpretation though.  I think "competency" and "proficiency" are virtually synonyms, and may used interchangeably in this context, but I won't argue that point with you. I expanded my answer a little bit and stand by it. Let's see how the votes come in! ;)

I personally don't care what the vote tally is.  I prefer to have someone with authority determine what the correct interpretation is.

Nor do I really, but I do put some stock in the credible opinions of peers.


text:
Why does entry into Class B specifically require clearance?
Entering Class B airspace requires an explicit clearance.  Entering Class C or Class D airspace requires only establishing two-way communications, upon which the pilot can still be explicitly refused entry.
So effectively, in both cases ATC maintains full control and can instruct a pilot not to enter their airspace. The only difference is whether the pilot asks for permission or simply announces an intent. This seems like a very fine semantic distinction which has no real-world meaning.
Why, then, does the FAA make this distinction? What practical impact does it have on a pilot?

I appreciate the word-smithing. I built upon them to tighten up the question just a little bit more.

For a large class B complex, the same approach control frequency may be working aircraft both inside and outside the B, with some of those outside the B in a class C or D.
Establishing communication with ATC is sufficient to enter the C or D unless the controller says no, but you still have to keep out of the B unless they say yes.  If the rule were the same for all 3 (i.e. you may enter the B unless specifically told no), then most initial contacts would get a reply to the effect of "N123AB, radar contact, remain clear of Bravo."  Lots of words used for lots of aircraft that may never intend to enter the class B.
The class C and D don't tend to need as much tight control as the class B, so "just" establishing communication is sufficient.  But just because the controller is talking to you, doesn't mean that they're okay with you entering the class B.
If we think about it in terms of computer logic, it'd be simple for the controller's first reply packet to include bits to indicate 'cleared into class B', 'cleared into class C', and 'cleared into class D', all of which are set as desired and any of which could later be changed as necessary.  But voice communication has nuances that digital processing lacks, and the amount of fast-moving (generally, airline) traffic in class B makes it desirable to have a more explicit clearance into that airspace required, and a stronger "stay out unless you have permission" requirement in place.

So the notion here is that Class B reached a tipping point where there is so much traffic, and so much detailed information required about said traffic, that ATC needed a way to initiate communication without de facto permitting traffic to proceed?

Yeah, fair summary. They may want details (aircraft type - i.e. aprox speed, intended routing, etc) that take some communications, and even then may delay giving clearance until traffic permits. Class B really is a different animal than C or D.

"the same approach control frequency may be working aircraft both inside and outside the B" — this is in no way unusual or any different from any other TRACON anywhere. Class C and Class D TRACON controllers are responsible for much more airspace than the charted C/D airspace. (In fact, Class B TRACONs are more likely to not extend much beyond the charted Class B airspace, at least the upper layer of it.)

The answer has to do with ATC's separation requirements.

In Class D airspace, VFR aircraft are not afforded any kind of separation. Mandatory traffic advisories and safety alerts are provided between VFR/VFR and VFR/IFR traffic.
In Class C airspace, VFR aircraft are provided target resolution laterally ("dots don't touch") or 500' vertically between them and IFR traffic. No separation is provided between VFR/VFR traffic; again, mandatory traffic advisories and safety alerts are provided.
In Class B airspace, all aircraft are provided explicit milage-based radar separation regardless of flight rules.

In order for controllers to reliably provide milage-based radar separation, aircraft must be operating predictably and with no room for deviation. Therefore VFR aircraft, which are not usually given explicit heading and altitude instructions (at least not usually at the same time), are given explicit clearances when entering Bravo airspace. This means the controller knows what the aircraft are doing and can vector traffic to effect the proper lateral separation.
This is also why cloud clearance requirements are relaxed to simply "clear of clouds" when in Class B airspace. Because ATC is providing radar separation, the risk of an IFR/IMC aircraft suddenly appearing out of a cloud and colliding with a VFR aircraft is minimal; instead, the only major concern is that a non-IFR-rated pilot does not enter IMC.
To make the comparison with Class C airspace clearer, observe that VFR pilots will often receive a heading assignment when operating in Class C airspace, especially on initial departure. They may also (or instead) receive the instruction to "Maintain VFR at or above/below [XXX altitude]." This is because ATC has a responsibility to maintain defined separation between IFR/VFR traffic. Because the separation standard is only target resolution, and because Class C airspace is much smaller than Class B, a full-blown clearance to fly a specific heading and altitude is not necessary.

The difference boils down to the likelihood that you’ll be in the way of IFR traffic—and how easily ATC can solve that problem.
A typical class C or D airport may see only a few hundred (C) or a few dozen (D) IFR operations per day, so the odds are you won’t be in anyone’s way when you pass through.  In the rare cases that you are, ATC can easily vector you to a different part of their airspace where you won’t be. So, the presumption is they’ll accept you.
In contrast, the biggest class B airports may see hundreds of IFR operations per hour, and while the airspace is much larger, there are so many planes in it there may be no way through that won’t put you in someone’s way. So, the presumption is they won’t accept you.
What this means to me as a VFR pilot is that I expect that I’ll be able to go through C/D, so that’s what I’ll plan. (It’s also so small and simple, it only takes a few seconds to replan off I’m wrong.) However, I assume I won’t be able to go through B, so I’ll plan my route around it and, if I happen to get a shortcut through, that’s just a bonus.

I agree with the mentality of approaching a B vs a C or D (although I've maybe had much better luck than you with transiting Bravos), but IMO the language of the FARs is too carefully chosen to be this open to interpretation. Without any evidence beyond a lifetime of experience with technical writing, I hold that there must be a very specific and concrete meaning which motivates the difference between the two.

@KennSebesta There’s also some holdover from the TCA/ARSA/TRSA rules when they adopted the alphabet system in the 90s, unlike other countries that wiped the slate clean, but I don’t see much room for interpretation here.

What I'm driving at is that from the pilot's perspective, a presumption one way or the other would be a very loose way to interpret these rules. I fly under a Class Bravo and presume I will always get in without any issue. If you presume the opposite, then it's fair to say the regulation is so loosely written that we are coming to contrary conclusions. Although that wouldn't be the first time this happens with the FARs, I hope to find a better explanation.

@Kenn, the "specific and concrete" difference is the separation services you're provided, as detailed in my answer.

Class B is expected to be a busier airspace. The difference between "ATC can deny clearance" and "you must get explicit clearance" is huge in that context.
Typically Class B surrounds major airports. If you just told ATC you're there and came on in, what if ATC was too busy to notice? The chances of a near collision incident or worse is quite high.
By requiring explicit permission to enter class B, the FAA is ensuring that the airspace controller knows who you are, where you are, and where you are going before you enter the airspace.
C and D are relatively lower traffic in general. In those airspaces, collisions are less likely and pilots are probably aware of any traffic announcing a position near them, so even if ATC drops the ball the pilots will call out any problem themselves.

This is a first attempt at an answer on Stack Exchange. I'm sure it's not ideal, but I'm happy to learn!

I see what you're saying, but I struggle with "If you just told ATC you're there and came on in, what if ATC was too busy to notice." Since the requirement for having established two-way comms is that ATC must respond, there's no slipping through this crack.

@KennSebesta. Not really; if the controller replies "N123AB squawk 1234 and ident" then you've "established radio communication" even though the controller still doesn't have you on his scope yet. (A reply, "aircraft calling, stand by" would not be established communication.)  But the fact that he's replied to you with your callsign doesn't mean that he has all the info he'd want/need to issue a Bravo clearance.  Sure, he could add "... and remain outside the Bravo" to lots of radio calls, but that much extra verbiage, over and over, becomes problematic after a while.

Or the controller could reply "N123AB, standby..."  That is established comms and you can come on in.

@KenSebesta. I get what you're asking but I wasn't sure how to make it more clear. Happily more experienced pilots (I'm a student in ground school) have provided more complete answers that hopefully answered your question fully. The accepted answer from Ralph J is excellent!

Welcome to Stack Exchange, madscience!


text:
How to sightsee in Class C and D airspaces?
There's a good explanation of procedures for Class B, all of which makes sense since pilots need specific clearance to enter Class B: How do I "sightsee" in a class Bravo airspace??
However, I wonder how much this changes for C and D. While a pilot does not need specific clearance to enter the Charlie and Delta, the local ATC can still direct the pilot to stay clear. This in effect means that ATC has the same control over Charlie and Delta airspace as in a Bravo. However, what isn't captured in the FARs is what kind of rationale ATC needs to refuse a pilot request, and if that changes based on the airspace.
In short, can ATC (reasonably) refuse sightseeing flights in Charlie and Delta?

You do need clearance to enter class C and D airspace.

@GdB No, you only need to establish and maintain two-way radio communication.

There isn't going to be a rule "ATC must have this kind of reason to refuse entry". Regulators absolutely don't want pilots saying "You don't have a good enough reason to refuse me entry to this airspace" and ignoring ATC. If they tell you to stay out, you stay out. If you don't like it, complain later.

@GdD In the US, clearance to enter C/D airspace is implicit when you establish two-way comms.

Right. The point is that if we're splitting hairs between having to ask permission vs declaring intentions which can be immediately overruled, it's really not a useful distinction. Maybe a follow-up question should be why the FAA makes a distinction between the two.

@DJClayworth there must be some established rule-book of some kind, as the gov't is specifically forbidden from restricting an individual's freedom in a capricious manner. The reason could be as simple as StephenS points out, which is that ATC doesn't have the time or confidence. But there must always be a reason, founded on the regulations, which can later be challenged in an appropriate forum.

@KennSebesta FWIW, I’d like to see that question.

@StephenS https://aviation.stackexchange.com/questions/87887/why-does-entry-into-class-b-specifically-require-clearance

@757toga I agree with everything you say. My point, perhaps badly made, was that, as the representatives of a gov't agency, they have to have a rulebook somewhere. The FAR doesn't give much insight into what the rulebook looks like and when we might be within our rights to say that a controller is not giving us access to airspace which we should otherwise be entitled to. It's a question borne purely out of curiosity, as I have personally never had anything other than a textbook exemplary experience with ATC.

The answer is the same in theory: you just tell ATC what you want to do, and they either approve it (possibly with some modifications) or deny it. Since all VFR services are on a “workload permitting” basis, which answer you get may depend on when you ask.
The only difference in practice is that class B airspace tends to be a lot busier than class C, which in turn tends to be busier than class D, so that gives you an initial indication of the odds they’ll be too busy to work you.
Your radio skills also play a part; a busy controller may still be willing to take on a pilot that sounds easy to work while rejecting those that sound clueless.
All that said, if you call ahead (on the phone) and explain what you want, they can often tell you when is the best time and how to word your request (or modify it) to improve your odds of a positive response.

Double +1 for the suggestion to call ahead.


text:
In a tailwheel plane, which design elements affect the propensity for a ground loop?
In comparisons between conventional gear vs. tricycle gear aircraft, it is generally explained that the CG location relative to the mains is a necessary condition for ground-looping. But there's very little more explanation than that.
I'd like to know more about the design traits which affect the propensity for a ground-loop. The following is an uninformed list of some things which I could see affecting a taildragger's ground handling, but I wouldn't hazard a guess as to relative importance.

CG location
Gear width
Gear shape
Gear springiness
Tire pressure
Tread traction

I would love to know the answer to this for improving my RC plane

TIL that R/C planes also had ground looping problems.

They have a stupendous number of problems in common with their larger brethren, at least as far as physics is concerned.  It's embarrassing how long it took me to join the EAA after the AMA, to learn from those with more at stake.

CG Fore/Aft Location
The further forward the main wheels are in front of the CG, the greater the propensity for ground looping.
Main wheel separation
The closer together the main wheels, the greater the propensity for ground looping.
Main wheel Toe-in and Toe-out
The following text and images come from the book "Landing Gear Design For Light Aircraft" by Ladislao Pazmany, obtained on-line via this web page on www.vincesrocket.com:

The "toe-in" and "toe-out" of a wheel is measured either in degrees or
in inches as shown in Figure 3-9.

How the "toe-in" or "toe-out" affects the directional stability and
control of an aircraft is a controversial subject.  The author
consulted with a number of aeronautical engineers and landing gear
experts.  The answers were mixed.  Some indicated that with "toed-out"
wheels as shown in Fig 3-10, the wheel on the outside of the turn
tends to run straight and counteract the ground loop.

Other opinions were that the "toed-in" wheels result in more drag (due
to skidding) in the outside wheel than in the inside wheel, therefore
counteract the ground loop as shown in Fig 3-11.

The recommendations in some Service Manuals are that a very small
amount of "Toe-in" (0 to 1/8 inch) measured at empty weight, is
desirable.  Essentially the wheels should run straight (zero toe-in)
at normal operating weight.  The PL-4A (Figure 1-11) designed by the
author has 0 degree toe angle and exhibits excellent ground handling.
Any tendency to ground loop initiation is easily countered with the
rudder and steerable tail wheel.  The same comments apply to tricycle
gear airplanes.

Here's a post from an on-line discussion forum for rc airplanes where the author describes a particular plane where the landing gear can be quickly removed and re-attached to convert toe-in to toe-out or vice versa.  The author notes that any toe-out seems to greatly increase the difficulty of keeping the aircraft going straight when the aircraft is rolling at high speed on the main wheels with the tail in the air.
The entire thread from which the above post was taken may be of interest-- the opening question is very similar to the question here: "What design aspects affect ground handling?"
Here's another post from another related thread on the same forum where the author-- a very experienced builder and flier of rc planes-- advocates some toe-in.
Camber
Camber is a significant factor.  If the bottoms of the main wheels are closer together than the tops, the wheels have positive camber.  If the tops of the wheels are closer together than the bottoms (like on a Me-109 or Spitfire), the wheels have negative camber.  Note that if a tailwheel aircraft has zero toe-in but positive camber when it is an a fuselage-level attitude, then the camber will automatically create toe-in when the tailwheel is on the ground.  If a tailwheel aircraft has zero toe-in but negative camber when it is in a fuselage-level attitude, then the camber will automatically create toe-out when the tailwheel is on the ground.  This may be part of the reason why negative camber has generally been found to contribute to an increased propensity towards ground looping, and positive camber has generally found to contribute to a decreased propensity toward ground looping.  Note also that when a ground loop begins and the "leading" or outside wheel starts to get more heavily loaded than the "trailing" or inside wheel as the plane starts to tip over toward the outside wingtip, negative camber will make the "leading wheel" tend to "dig in" or experience an increase in traction, exacerbating the process, while positive camber will tend to reduce this tendency.
Tire traction
Tailwheel aircraft show much less propensity towards ground-looping when landing on grass or wet pavement, than when landing on dry pavement.  This might seem to suggest that any tread on the tires of the main wheels--or at any circumferential grooves -- should increase an aircraft's propensity for ground looping, by increasing traction.  However this related ASE answer indicates that tread or circumferential grooves don't actually increase a tire's grip on dry pavement.
It would seem that anything that increases a main wheel's "contact patch" -- which would include decreasing the tire pressure-- would increase the ground-looping tendency on dry pavement.
CG height
A high CG location increases an aircraft's tendency to tip over and drag the outside wingtip during a ground loop.  Even before the wingtip touches, any tipping will load up the "leading" wheel and unload the "trailing" wheel, which will exacerbate the developing ground loop.
Fuselage length
An aircraft with a relatively short fuselage is called "short-coupled", and will experience a greater propensity for ground-looping than an aircraft with a relatively long fuselage.1  If the mass of the aircraft is concentrated near the CG so that the moment of rotational inertia is low, the aircraft will have less tendency to ground loop than if the mass of the aircraft is distributed far from the CG.
Annex
Here is a PDF from a course on landing gear design.  Note however that it doesn't go into a great deal of detail.  Toe-in/out and camber are not discussed.
Footnotes:

This sentence naturally begs the question-- exactly how might we quantify how "short-coupled" an aircraft is?  Probably not by comparing the fuselage length to the width of the landing gear tread, as this would seem to imply that increasing the distance between the main wheels is not helpful.  There may be some room for further refinement of this part of this answer--

This is good info! One thing I wonder about is  "The closer together the main wheels, the greater the propensity for ground looping." Gliders are an extreme case of this, where the wheels converge to the maximum extent, i.e. there's only one of them centered in the fuselage. Gliders have almost zero likelihood of a ground loop, so long as the pilot don't drag a wingtip. Thoughts?

This is an excellent answer. @quiet flyer, all its missing is sources for some of the sections. Any chance you could get a source link into each major block?

@KennSebasta -- some of it is just kind of common knowledge that's widely available-- I'll think about seeing if I can find a some sources--

Note that a similar toe-out strategy has sometimes been used on cars as well, and for almost the same reason - this is what causes the car to revert itself back to going straight if you take your hands off the wheel while driving.  (Obviously this is not the case with newer cars with lane-keeping or self-driving features.)

@quietflyer, thanks. It would be especially good to see if a toe-in/toe-out consensus has formed since Pazmany published his book 40 years ago. The back and forth on that topic suggests that even the most fundamental questions about ground looping remain unresolved.

Addressing only why gliders, even with only one main wheel, rarely ground loop.
Point by point:

No toe-in/-out.
No camber.
Tailwheel only, and often landing on grass.
Low CG.  The wheel's axle may be almost as high as the CG.
Mass concentrated very close to the CG, especially if still carrying ballast.

Large wingspan and lightweight wings make it easier to keep the wings level with ailerons even late in the rollout,
so by the time a wingtip drags, groundspeed is too slow to cause damage.


text:
What noise reduction measures were taken in the time between hush kits and chevrons?
In older jets such as 727 hush kits are used in the engines to reduce noise. In very modern jets such as 787 or 747-8F, chevrons are used for the same purpose. What happened in between? The switch to high bypass ratio turbofans alone is sufficient an improvement in engine noise that no special noise reduction device is required? (Not even hush kits?)
A hush kit:

A chevron:

Treatments to reduce noise tend to also reduce efficiency, so if the noise is already low enough it's not worth the cost.

Also: Do nacelles around aircraft engines help in reducing the engine sound?

Generally speaking one is an add on and the other is integrated into the design. Over the years the FAA has implemented progressively more stringent noise regulations both for aircraft that are flying as well as to the full certification process. The changes came in stages and have been progressively altered for the past 30 or so years take a look at §36.103   Noise limits for the current regulation .  You can find some of the FAA legislation here and the full text of the relevant FAR's here. As such newer engines will incorporate noise mitigation directly into the design as they need to pass the certification process. However in cases relating to older designs it may be cost effective to design an attachment (like a hush kit) that can be added to an existing engine to comply with regulations since the engine was certified at a time that the regulations were far more relaxed. Due to the letter of the law planes can be grounded for being non compliant so a hush kit may be cheaper than a new plane. For example this was the case with the Lear 23 which had required hush kits.
Interestingly at least one patent for a hush kit stresses that it is for an existing engine design.

BACKGROUND OF THE INVENTION
The present invention relates generally to
  jet aircraft turbine engines and, more specifically, to a hush kit for
  use on an existing engine and airframe.

You can similarly find the patent for the chevron here. Which describes it more as part of the engine than as an add on.

Yes indeed - the higher the bypass ratio, the lower the noise:

Noise is a function of the velocity of the accelerated air stream. For a given thrust, accelerate more air to a lower velocity and noise goes down (and efficiency goes up).
The bypass air is much slower than the turbine exhaust air, and surrounds it like a tube.

The hush kits were for previous generation noisy aircraft. I used to live and work near an airport, and could hear and see hush kitted aircraft take off. Caravelles, Iljoesjins, Tridents, made you look up and observe the pile of black smoke propelling the aircraft. Later, stricter noise regulations allowed only hush kitted screamers to take off, quieter but still smoking.
Article

Noise is proportional to exhaust velocity to the power 6 according to NASA

Great research program, thx., It shows a very omplicated function of frequency, exhaust gas velocity, distance, and flight angle. Have amended the answer.